With the further increasing costs of gas and fuel oil, combination with an ever increasing social consciousness directed to conservation and recycling, further attention is being directed to alternate fuel sources such as wood and other solid/semi-solid combustibles or combustible residues. A resurgence of interest and attention is being given to biofuels, biofuel combustion processes and biofuel combustion devices.
“Biofuel” is a term generally understood to embrace any fuel derived from biomass, namely, recently living organisms or their metabolic byproducts. Agricultural products specifically grown for use as biofuels include, among other things, corn, soybeans, flaxseed, rapseed and hemp. Furthermore, biodegradable outputs from industry, agriculture, forestry, and households, e.g., straw, timber, manure, sewage and food left-overs, can also be used to produce “bioenergy.”
Heaters or stoves, more generally, “heating appliances,” for burning biofuels are known to provide acceptable alternative heat sources for conventional heating units such as gas, electric and oil furnaces. Biomass pellets of a variety of compositions are well known fuel sources, as are cereal grains such as corn and wheat, to name but a few.
While many perceive pelletized fuel sources as being especially advantageous due to size uniformity and low moisture content, efficient energy producing combustion nonetheless requires attentive regulation of a variety of combustion parameters, for example, and without limitation, draft regulation, backfire prevention, thorough fuel conversion, and ash management/conditioning. In light of, among other considerations, increasing costs for pelletized biofuels due to increasing, and in some places, unmet demands for same, it is believed especially advantageous and desirable to utilize raw biomass fuel sources such as grains, which generally are readily available, and favorably priced on a per unit of energy produced basis.
Shortcomings and/or challenges associated with efficient biofuel combustion have been and continue to be well documented. For instance, U.S. Pat. No. 7,004,084 (Anderson et al.), the contents of which are incorporated herein by reference, identify a variety of challenges and, as the case may be, heretofore known approaches to those challenges, namely those of: fuel delivery (1:36 et seq.); initial fuel ignition and start-up (1:60 et seq.); clinker formation (2:34 et seq.); and, thermal operational optimization (2:48 et seq.).
In connection to fuel delivery, heretofore known pellet burners, corn stoves, etc. deliver, as by a direct auger dump, a mass of fuel at a select rate into a burn cup or fire pot (see e.g., U.S. Pat. No. 4,947,769 (Whitfield), U.S. Pat. No. 5,001,993 (Gramlow), U.S. Pat. No. 5,285,738 (Cullen), U.S. Pat. No. 5,488,943 (Whitfield et al.), and U.S. Pat. No. 7,004,084 (Anderson et al.). With reference to the subject teachings, whether the fuel is received in a cylindrical “cup” or elongate “pot,” its ingress path is unencumbered; a straight shot from a direct dump orifice, generally overlying the cup or pot, to a fuel support structure or platform of the cup or pot (but see Gramlow '993 which shows fuel entry via a cup sidewall). For the most part, as the dumped fuel is received upon the fuel support structure, a fuel pile or concentrated mass of fuel forms thereon. As is common practice in elongate burn box arrangements, fuel enters from one end of the box, i.e., a narrow box end, and proceeds toward the opposing end while being consumed.
Anderson et al. '933, having flagged the delivery of corn kernels to the combustion chamber per se as a challenge, provide a pneumatically enhanced augered delivery of corn. Via a specially configured auger fuel feed tube, controlled air delivery thereto, pegged to a condition of a fuel agitator positioned within the ignited fuel, permits the fuel to be deposited proximal to the auger orifice during a low flow condition, or distal of the auger orifice during a high flow condition. In-as-much as the subject fuel delivery methodology is alleged to yield performance improvements, technician calibration and adjustment of such delivery system is a prerequisite.
In connection to fuel and/or ash conditioning, heretofore, burn pots have been equipped with a variety of mechanized arms and the like so as to “sweep” or otherwise direct spent fuel for discharge from the burn pot (see e.g., Whitfield ′796, Cullen ′738, or Whitfield ′943), or to stir and mix materials in the bottom of the fire pot so as to maintain uniform combustion, remove ash and prevent clinker formation (see e.g., Gramlow ′933 and Anderson et al. ′933). Presently available commercial combustion appliances tend to feed fuel into a first end of a burn box via an auger, whereupon the entire mass of the fuel progresses towards the opposing end of the box while being mixed or stirred via an auger extension or the like to ensure complete fuel combustion, burning as it goes, with spent fuel exiting the chamber, commonly by dumping into an ash drawer, ash, clinkers and all. As heretofore known burn pots are intended to efficiently or optimally process fuel of a particular single or uniform character, e.g., pelletized, or minimally processed, or raw biomass, “optional” fuel specific burn pot assemblies, in the form of retrofit kits, are required and offered in the market place, see e.g., the Harman PC 45 Corn/Pellet stove.
Shortcomings with heretofore known burn box fuel agitators are believed twofold. First, the agitators are completely overburdened, or even surrounded by a mass of ignited, burning biofuel and/or a flame, and thus reach very high temperatures. In such environment, fuel impurities form deposits within the burn box which coalesce into a glass-like coating, necessitating periodic maintenance (i.e., removal and cleaning) for attainment of sought after thermal energy production. Second, the agitators seldom have an advantageous span to “reach” above or beyond the clumpy pile of ignited, burning biofuel. As a result, partially burned fuels create a “dome” above the fire that hinders, and often times prevents fresh fuel from entering the fire zone, a requisite for efficient operation.
In connection to initial ignition, start-up and thermal operational optimization, although incremental improvements have arguably been made, there remains ample room for improvement (see, e.g., Applicants' copending U.S. patent application Ser. No. 11,550,500, filed Oct. 18, 2006, entitled “Process Control Methodologies for Biofuel Appliance,” incorporated herein in its entirety). Whether due to confinement in a “deep” cup or pot, localized accumulation in the pot per se, or owing to inadequate initial fuel delivery of post delivery fuel distribution and/or ash management, heretofore known combustion appliances necessarily produce a characteristic concentrated or narrow industrial flame, i.e., a “bunsen burner” look and feel. Furthermore, such devices are generally limited in their dynamic range, i.e., their ability to run at both very high and very low heat outputs, even when utilizing a single, uniform fuel source, e.g., a pelletized biomass, let alone when the biofuel may be variable from one heating event to another.
In light of the foregoing state of the art, and improvements or improved features in and of the new, i.e., last generation of biofuel heaters/heating appliances, there nonetheless remains great room for improvement, especially in the arena of non-industrial applications. It remains highly desirable to provide an apparatus which can, for, all practical purposes, efficiently operate with no on site calibration, modification, alteration, upgrade, retrofit, etc., and further still, provide an apparatus which can readily process a variety of biomass feed stocks as fuel, i.e., either or any of pelletized biomass, semi-processed biomass, or raw biomass, separately, or in combination. Furthermore, it remains desirably and advantageous to more efficiently handle fuel distribution and management, as well and improve upon heretofore known ash conditioning or management techniques. Finally, there remains a need to eliminate ignition and start up shortcomings, and provide a combustion process which is less dependent upon the plurality of heretofore adjustments in relation to one or more of fuel feed type, character or quality, fuel feed rate, and combustion air dynamic, flow and character.